Process and composition for anodizing a tincoated article

ABSTRACT

In a method of forming an anodized coating on the surface of a tin-coated ferrous article, the article is subjected to anodic electrolysis in an aqueous electrolyte comprising a dialkali metal phosphate, an alkali metal carbonate and an alkali metal hypophosphite.

Unite States atent Richard G. Snyder Bethlehem, Pa.

May 16, 1969 Oct. 26, 1971 Bethlehem Steel Corporation Inventor Appl. No. Filed Patented Assignee PROCESS AND COMPOSITION FOR ANODIZING A TIN-COATED ARTICLE 14 Claims, 2 Drawing Figs.

U.S. Cl 204/56 R Int. Cl C231) 9/00 Field of Search 204/56;

Primary Examiner-John H. Mack Assistant Examiner-R. 1.. Andrews ABSTRACT: In a method of forming an anodized coating on the surface of a tin-coated ferrous article, the article is subjected to anodic electrolysis in an aqueous electrolyte comprising a dialkali metal phosphate, an alkali metal carbonate and an alkali metal hypophosphite.

PROCESS AND COMPOSITION FOR ANODIZING A TIN- COATED ARTICLE BACKGROUND OF THE INVENTION In the manufacture of tin-plated baking pans for use in the commercial baking industry, it is desirable to supply a nonreflective dark oxide coating, or film, to the exterior surface of the pan in order to enhance radiant heat absorption. Such a coating provides a uniform, desirably brown crust on bread or other baked products. In the absence of the dark oxide film, the highly reflective tinplate pan surface produces a spotty, light colored (undesirable) baked product crust. The dark filmed pan, with its heat absorptive surface, also reduces baking time, and consequently increases oven capacity.

At one time, hot dip tinplate was used exclusively in manufacturing commercial baking pans, and processes have been developed by which the tinplate, either before or after fabrication of the pan, is anodized electrolytically to form the dark tin oxide coating, or film.

As the tinplate industry has gradually converted from the hot dip tinning process to the more rapid and efficient electrolytic process, the baking pan industry has shifted to utilization of electrolytic tinplate. Use of electrolytic tinplate for the manufacture of baking pans, especially fused, or flowbrightened, tinplate, which is desired for its smooth surface, has led to inconsistent results in the anodizing of the tinplate. It is well known that hot dip tinplate is generally less porous than flow-brightened electrolytic tinplate. This is particularly so with tinplate in the heavier tinplate gages (up to 28 gage), and even more so in the so-called sheet metal gages (27 gage or heavier). Baking pans are usually made from sheet metal gage material in the range of 27 to 20 gage. The coating weight of the tinplate is normally about l.5 pounds of tin or greater per base box (a base box is equivalent to 435.56 sq. ft. of coated surface). Generally, it is more difficult to obtain the desirable dark anodized tin oxide film on electrotinplate than on hot dip tinplate. Poor anodizing, resulting in a thin tin oxide film, is attributed to exposure of iron or iron-tin alloy. Such exposure may also result from deep scratches, giving rise to a thin oxide film adjacent the scratch. Prior methods of anodizing have not proved satisfactory for the production of a consistently uniform oxide film for all gages of electrolytic tinplate, or on badly scratched tinplate.

Accordingly, a principal object of this invention is to produce an adherent, continuous and uniform tin oxide film on either hot dip or electrolytic tinplate, which will develop an acceptable coloration upon being subjected to elevated temperatures.

SUMMARY OF THE INVENTION l have found that anodizing of tin-coated ferrous articles can be performed effectively, even when the tinned surface has small exposed areas ofiron or iron-tin alloy, by employing an aqueous electrolytic bath containing hypophosphite ion in conjunction with phosphate and carbonate ions at an operating pH greater than 8.

In accordance with this invention, a tin-plated article in the form of a flat sheet, or a fabricated article such as, for example, a baking pan, is subjected to an anodic treatment in an aqueous electrolyte of a dialkali metal phosphate, an alkali metal carbonate and an alkali metal hypophosphite in the approximate respective ratio of :5:2, in which the phosphate compound may range from 10 grams per liter upward. The duration of the treatment, performed at a low current density, will depend on the thickness of the tin oxide coating desired. Articles for which this invention has particular applicability, such as tin-plated baking pans, are generally given a tin oxide coating of a thickness of 10 microinches or greater. A film thickness of this order requires an anodizing treatment offrom l to 3 minutes. The resultant tin oxide (stannous-stannic oxide) film is in the metastable form, usually having a color ranging from yellow to red. Once anodized, the article, in the case of baking pans, is coated with a releasing agent, such as silicone of the thermosetting polysiloxane type. The article is then heated to cure the silicone, the heat treatment at the same time converting the tin oxide to the stable form. After the heat treatment, the tin oxide coating assumes a desired greenish-grey color.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 represents the sequence of interference colors versus current density applied for 1 minute for an unacceptable anodized coating at 20 amperes per square foot, as determined by the Hull cell with a known electrolyte.

FIG. 2 represents the sequence of interference color versus current density applied for 1 minute for an acceptable anodized coating at 20 amperes per square foot, as determined by the Hull cell with the electrolyte of this invention.

DETAILED DESCRIPTION In one specific embodiment, the article subjected to the anodic treatment of this invention was a 25 gage cold rolled sheet steel having an electrolytic tin coating deposited thereon, with the coating flow-brightened by melting the tin after electrodeposition. The deposited tin had a thickness of microinches, representing l.5 pounds of tin per base box. The tin-plated article had, prior to the anodizing treatment, a relatively uniform surface, as judged by visual inspection prior to anodizing. However, certain areas of the pan contained fabrication scratches showing bare steel substrate. Microscopic inspection also revealed a certain amount of porosity characteristic of this type of tinplate. The article had been fabricated into a bread pan from a flat sheet of tinplate.

The pan was immersed in a aqueous electrolyte of 50 grams per liter (g./l.) disodium mono-hydrogen phosphate heptahydrate (Na,HPO,-H,O), 25 g./l. anhydrous sodium carbonate (Na CO and I0 g./l. sodium hypophosphite monohydrate (NaH,PO,H,O), wherein the pan, acting as anode, was subjected to electrolysis at a current density of 20 amperes per square foot (asf.) for 3 minutes. The pH of the bath was maintained at 10.3. The bath temperature was about F.

Upon completion of the electrolyzing step, the pan was withdrawn from the bath, rinsed with water and dried in an oven at 300 F. for l5 minutes. The pan had a uniform film of yellowish red tin oxide over its entire surface. The oxide thus formed is a meta-stable stannous-stannic oxide which requires a heat treatment to convert the oxide to the stable form. However, the meta-stable tin oxide film is sufficiently adherent to the tinplate substrate so that the as-formed anodized product can be stored prior to the subsequent steps of applying and curing the releasing agent and stabilizing the oxide film.

In the case of the articles of the above example, the dried, anodized pan was coated with a thin film of silicone releasing agent, and the silicone-coated article then subjected to a temperature of approximately 425 F. for 2 hours. The heat treatment performed the dual purpose of curing the silicone, thus transferring the elastomer into an adherent, long wearing surface, while at the same time converting the tin oxide into the desired stable form of grey-green oxide. Heat-cured silicones of the polysiloxane type will withstand temperatures well within those used in commercial baking operations, while the stable tin oxide exhibits no physical change at baking temperatures.

Alternatively, the anodized article bearing the metastable tin oxide coating can be heat treated at a temperature of ap proximately 400 F. to convert the oxide to the stable form, applying and curing the silicone at some later period. However, in most instances, the economics of the process will favor conversion of tin oxide and curing of silicone in one operation.

In any event, the anodized surface must be converted to a stable form of tin oxide before the anodized article is ready to be placed in service.

While the silicone acts as releasing agent in the pan interior only, normally, when the silicone is applied after pan fabrication, the entire pan is spray coated with silicone.

Generally, the anodizing step and the application of silicone are performed on the fabricated pan, as in the above example. However, in some instances, it may be desirable to anodize and apply releasing agent to the tinplatein sheet form.

As a measure of expected service life, five specimens of tinplated strip of the same gage and coating thickness as that used in the article of the example were anodized in the same manner as described above, and, with the silicone coating omitted, heated to stabilize the oxide coating. A disc, having a diameter of 2% inches, was cut from each of the strips and exposed to distilled water at room temperature for 15 hours. No visible rusting appeared on any of the test specimens.

The efficiency of the improved process and bath composition of this invention over prior practice for anodizing electrolytic tinplate is clearly shown in the drawing. FIG. 1 of the drawing represents Hull cell plating range tests made from a prior tin anodizing bath comprising an aqueous solution of I g./l. disodium hydrogen phosphate heptahydrate and 50 g./l. sodium carbonate. FIG. 2 represents Hull cell plating range tests made from an aqueous bath comprising 100 g./l. disodium hydrogen phosphate heptahydrate, 50 g./l. sodium carbonate and 20 g./l. sodium hypophosphite monohydrate. A complete description of the Hull cell and its operation is given in Metal Finishing Guide Book for 1969 (published by Metals and Plastics Publications, Inc.) at pages 420-423.

The Hull cell tests were performed on 2 /4-inch by 4-inch panels, using a constant current of 2 amperes for 1 minute. The cathode was a steel sheet. A ceramic Hull cell was used so that the test solutions could be heated on a hot plate during the test. Tests were performed at 65 C. The test specimens were made from flow-brightened electrotinplate produced on a commercial continuous electroplating line. These specimens were of the type found to be difficult to anodize, by prior practice. The current density was varied from 1 to 100 asf. across the panel, and a pattern of interference colors relating to the thickness of oxide film was obtained. The yellow-red interference color is a measure of satisfactory film thickness. Hence, the current density at which this color is first formed is the lowest current density at which the bath will operate for an electrolysis period of 1 minute. Following the first occurrence of the yellow-red color, which occurs in the second order of interference, the yellow-red color sequence then repeats itself in the third order, and as the coating becomes thicker, alternate red and green oxide films are produced with each order.

Referring to the drawing, FIGS. 1 and 2 represent'film buildup on electrolytic tinplate of a type which is generally recognized as difficult to anodize. It will be noted that in the bath employed in this invention, represented by FIG. 2, the yellow color of the yellow-red band denoting the onset of a desired metastable tin oxide film first appears at about 8 asf., and good definition of the yellow-red band is noticeable between 2 to asf., with a uniform buildup of film from there to approximately 100 asf. On the other hand, the known anodizing bath, formerly used on hot dip tinplate, but unsuccessful on certain types of fused electrotinplate, and represented here by FIG. 1, did not produce a yellow-red color until the current density was increased to approximately 100 asf. for the same time period.

A current density of 100 asf. is usually too high for practical commercial operation, due to excessive gassing at the electrodes, and the accompanying voltage required to produce this high current density. Generally, the current density of the anodizing process should be below 50 asf. A desirable operating range is between and 30 asf. for l to 3 minutes. However, if high current densities are used or normally occur on some parts of the article being anodized, the method of this invention is adequate to anodize the entire surface effectively.

Bath temperature may range from room temperature to 185 F.

In addition to the bath of the foregoing examples, which contained 100 3.1]. sodium dibasic phosphate, a series of tests was made in which the dibasic phosphate concentration was 10, 25, 50, 100 and 200 gJl. and saturation. These tests were conducted primarily to establish the optimum relationship of the three solutes in the bath, i.e., phosphate, carbonate and hypophosphite. All tests in this series were made on fused electrolytic tinplate of the type ordinarily considered difficult to anodize. Good anodizing results were obtained for each concentration of dibasic phosphate compound. The bath is also operable at 5 g./l. of the phosphate. However, commercial control at low bath concentration would be a problem.

The dibasic phosphate can be supplied directly from its salts or prepared indirectly from stoichiometric quantities of trisodium phosphate and phosphoric acid, or sodium hydroxide and phosphoric acid.

In the range of 5-200 g./l. dibasic phosphate, and up to saturation, it was found that a ratio of about 42321.4 for the 20 phosphate (POf): carbonate (C0 hypophosphite(ii- P0,

ions (radicals) produces the optimum bath relationship. However, the baths are operable when the carbonate component is somewhat lower than the amount shown by the ratio given above, as long as there is sufficient carbonate present to produce a pH greater than 8, preferably 10 to ll. Excess amounts of carbonate are not seriously detrimental, for under such conditions the pH will remain at the desirable range of 10-11. Furthennore, the hypophosphite component can be somewhat lower than the amount shown by the ratio given above and still be effective, but the hypophosphite should not be much above the ratio of 3. With a phosphatezcarbonatezhypophosphite ratio of about 4:3: 1 .4, and allowing for the foregoing variations from this ratio, the concentration range for the anions in the bath will be as follows:

Phosphate (PQf)- 2-90 grams/liter Carbonate (C053) l.5-70 grams/liter Hypophosphite (H PO,)0.5-$0 grams/liter The level of bath concentration desired, within the dibasic phosphate range given, will depend somewhat on the available voltage supply.

The recommended procedure in preparing the bath is to add the hypophosphite last, in small increments with stirring.

While baths below 50 gJl. phosphate produced slightly duller films than those above 50 g./l., this dullness is not objectionable as subsequent glazing and baking of the film transforms the film into a glossy stable oxide of the proper color and appearance.

The particular salts given in the example are preferred for optimum operation of the bath, although other compounds may be substituted as long as the bath comprises the anions of phosphate fliQj) and hypophosphite (H POQ), and an alkali which maintains the pH between 8 and l 1, Xor slightly greater. For example, sodium hydroxide (NaOl-l) may be substituted for sodium carbonate. Sodium hydroxide will produce a higher pH than the carbonate, and may require closer control, as the carbonate, either alone, or in conjunction with sodium hydroxide, and when added in sufficient quantity, maintains the pH in the preferred range of between 10 and l 1.

Other phosphates may be used, including the meta and pyro phosphates. in addition, other alkali metals, notably potassium, can be substituted for the sodium of the bath components of the example, although the sodium compounds would generally be favored because of their availability.

The bath of this invention has exceptionally good stability. For example, a bath containing g./l. dibasic phosphate, 50 g./l. carbonate and 20 g./l. hypophosphite was subjected to a current of 30 ampere-hours per gallon of bath solution at 65 C. and a current density of 20 asf. The decrease in sodium hypophosphite content was less than 5 percent of the amount originally present.

It has been the practice with some pan manufacturers when faced with electrolytic tinplate difficult to anodize, to overcoat, preliminarily, the tin-plated sheets with additional tin in order to render the sheets amenable to the anodizing process. By employment of the anodizing process of this invention, tin overcoating of the electrotinned article is unnecessary.

Another feature of the process and bath of this invention is in the fact that either hot dip or electrolytic tinplate, or the two jointly, can be anodized from the same bath at the same low current density and short anodizing time dictated by commercial operations.

lclaim:

1. The method of anodizing a tin-coated ferrous article which comprises subjecting said article as anode to electrolysis in an electrolyte comprising an aqueous solution of a phosphate in which the phosphate radical is present in an amount between 2 and 90 grams per liter and a hypophosphite in which the hypophosphite radical is present in an amount between 0.5 and 50 grams per liter at a current density between about 8 and 100 asf. for a time sufficient to form a continuous, uniform tin oxide coating on the surface of said anicle, said electrolyte having a pH between 8 and l 1.

2. The method according to claim 1 wherein the pH is maintained at between 8 and l l by the presence in the electrolyte of alkaline compound of the group consisting of a soluble carbonate or hydroxide.

3. The method according to claim 1 wherein the tin oxide coating is a metastable tin oxide.

4. The method according to claim 2 wherein the phosphate is a dialkali metal phosphate and the hypophosphite is an alkali metal hypophosphite.

5. The method according to claim 2 wherein the alkaline compound is a carbonate.

6. The method according to claim 4 wherein the alkaline compound is a carbonate.

7. The method according to claim 6 wherein the carbonate is an alkali metal carbonate.

8. The method according to claim 7 wherein the carbonate radical is present in an amount between L5 and 70 grams per liter.

9. The method according to claim 8 wherein the phosphate, carbonate and hypophosphite radical are present in the electrolyte in the respective approximate ratio of 4:3: 1 .4.

10. The method according to claim 8 wherein the phosphate is a disodium monohydrogen phosphate heptahydrate, the carbonate is anhydrous sodium carbonate and the hypophosphite is sodium hypophosphite monohydrate.

11. The method according to claim 8 wherein the anodized article resulting from electrolysis is heat treated at a temperature and for a time sufficient to produce a stable oxide of tin.

12. The method according to claim 8 wherein the current density is between 12 and 50 asf.

13. An electrolyte for anodizing tin-coated ferrous articles which comprises an aqueous solution of a phosphate in which the phosphate radical is present in an amount between 2 and grams per liter, a carbonate in which the carbonate radical is present in an amount between 1.5 and 70 grams per liter, and a hypophosphite in which the hypophosphite radical is present in an amount between 0.5 and 50 grams per liter.

14. A composition according to claim 13 wherein the phosphate is a dialkali metal phosphate, the carbonate is an alkali metal carbonate and the hypophosphite is an alkali metal hypophosphite.

B i i t i 

2. The method according to claim 1 wherein the pH is maintained at between 8 and 11 by the presence in the electrolyte of alkaline compound of the group consisting of a soluble carbonate or hydroxide.
 3. The method according to claim 1 wherein the tin oxide coating is a metastable tin oxide.
 4. The method according to claim 2 wherein the phosphate is a dialkali metal phosphate and the hypophosphite is an alkali metal hypophosphite.
 5. The method according to claim 2 wherein the alkaline compound is a carbonate.
 6. The method according to claim 4 wherein the alkaline compound is a carbonate.
 7. The method according to claim 6 wherein the carbonate is an alkali metal carbonate.
 8. The method according to claim 7 wherein the carbonate radical is present in an amount between 1.5 and 70 grams per liter.
 9. The method according to claim 8 wherein the phosphate, carbonate and hypophosphite radical are present in the electrolyte in the respective approximate ratio of 4:3:1.4.
 10. The method according to claim 8 wherein the phosphate is disodium monohydrogen phosphate heptahydrate, the carbonate is anhydrous sodium carbonate and the hypophosphite is sodium hypophosphite monohydrate.
 11. The method according to claim 8 wherein the anodized article resulting from electrolysis is heat treated at a temperature and for a time sufficient to produce a stable oxide of tin.
 12. The method according to claim 8 wherein the current density is between 12 and 50 asf.
 13. An electrolyte for anodizing tin-coated ferrous articles which comprises an aqueous solution of a phosphate in which the phosphate radical is present in an amount between 2 and 90 grams per liter, a carbonate in which the carbonate radical is present in an amount between 1.5 and 70 grams per liter, and a hypophosphite in which the hypophosphite radical is present in an amount between 0.5 and 50 grams per liter.
 14. A composition according to claim 13 wherein the phosphate is a dialkali metal phosphate, the carbonate is an alkali metal carbonate and the hypophosphite is an alkali metal hypophosphite. 